Plastics have found ever increasing use as replacements for glass and metal containers in packaging, especially of foods and beverages. The advantages of such plastic packaging includes lighter weight, decreased breakage versus glass and potentially lower costs. However, the gas-barrier properties of even reasonably good barrier plastics such as polyethylene terephthalate and polyvinylchloride are inadequate for many purposes. This inadequacy is particularly acute with small containers of 1/2 liter or less in volume where the surface to volume ratio is relatively high, such that many food products do not have an acceptable shelf life. Thus, the most widespread use of polymeric containers has been in the 1 to 2 liter size, but even with these larger sizes shelf life is limited, and there is considerable value in reducing the permeability of such containers. Permeation of gases through the walls leads to product degradation in the form of decarbonation (carbon dioxide loss) from carbonated drinks and to oxygen ingress and taste change in beers and other food products.
Barrier coatings have been applied onto plastic containers in order to reduce their permeability. Not surprisingly, it has heretofore been thought necessary to maximize the area of the container that is coated with these barrier materials in order to achieve satisfactory results. U.S. Pat. No. 4,515,863 (Cobbs et al.) discloses the coating of an entire polyethylene terephthalate (PET) bottle uniformly with polyvinylidene chloride (PVDC). U.S. Pat. Nos. 4,534,995 (Pocock et al.), 4,525,377 (Nickel et al.) and Japanese Kokai 60-2361 (Kuraray Co., Ltd.) all disclose pre-coating PET parison preforms with PVDC or with ethylenevinylalcohol (EVAL) barrier materials prior to blow molding bottles. In every case these parisons are coated right up to the neck thread area, whereby the final blow molded bottle has virtually its entire external surface coated. U.S. Pat. No. 4,478,874 (Hahn) discloses vapor deposition of inorganic oxide barrier coating to the entire external surface of PET containers and bottles.
Polymeric containers are low cost products and therefore are very sensitive to the costs of manufacture. As a result, the industry is constrained from using barrier coatings at substantial thicknesses to achieve major reductions in permeability. Furthermore, blow molded polymeric containers usually have wall portions in which the plastic is of non-uniform thickness, such that some wall portions allow less permeation than other portions. Thus, uniformly applied barrier coatings may be wastefully applied to thick areas that already have relatively low permeability. It has also been discovered that applying some barrier coatings to entire container tends to exacerbate stress cracking problems in small radius areas in the base portion of some container shapes.
It would be desirable to make more economical and effective use of barrier coatings for polymeric containers while avoiding detrimental side effects.